1. Field of the Invention
The present invention relates to a frequency band extending apparatus, a frequency band extending method, a player apparatus, a playing method, a program for causing a computer to execute reproducing processing, and a recording medium on which the program is recorded, all being capable of playing encoded data, which is encoded after deleting high frequency band, with higher sound quality.
2. Description of Related Art
In recent years, music distributing services, which provide encoded data such as MP3 (International Standard ISO/IEC 11172-3, MPEG Audio Layer 3), have become increasingly popular. In most of these services, encoded data whose bit rate is reduced is distributed so that it does not take time in downloading the data.
Encoded data of a low bit rate is often encoded by deleting component of signal belonging to high frequency band of 15 kHz or more, which is hardly audible by human ears. As a result, the data becomes small in file size. However, the deletion of the high frequency band signal leads to issues such as loss of “realism” which would otherwise be provided by an original signal and muffled sounds.
To cope with these issues, in an encoding system, such as HE-AAC (International Standard ISO/IEC 14496-3, High Efficiency MPEG4 AAC), a band extension technology is used to generate component of signal belonging to a higher frequency band of about 15 kHz or more, to thereby reproduce higher frequency component close to an original signal. Also, in recent years, a post-processing band extension technology or the like is employed to reproduce higher frequency component close to an original signal. In this technology, a signal obtained by subjecting data, which is encoded by deleting component of signal belonging to a higher frequency band, to decoding processing is inputted and the higher frequency band is interpolated.
For example, in a technique proposed in Japanese Patent Application Publication No. JP 2004-184472 (Patent Reference 1), a high frequency band signal is generated by mixing an input signal with a local oscillation signal, and the band is interpolated by adding the input signal and a higher frequency component filtered with a passband characteristic depending an encoding system or a type of music. In a technique proposed in Japanese Patent Application Publication No. JP 2002-175092 (Patent Reference 2), for attaching the higher frequency band signal component, an input signal is transformed by Fourier transformation into the frequency domain, an envelope for a higher frequency band is estimated from a frequency spectrum of a low frequency band, and a gain of the frequency spectrum of the low frequency band is adjusted so as to fit the envelope.